Steel in elevated temperature service under stress



Patented Nov. 13, 1945 UNITED STATE 'sTEEL 1N ELEVATED TEMPERATURE .SERVICEYUNDER STRESS Marcus A. Grossmann, Chicago, Ill-., and Richard F. Miller, Pittsburgh, Pa., assignors of one-half to Carnegie-Illinois Steel Corporation, a corporation of New Jersey, and one-hall. to United States Steel Corporation of Delaware, a corporation of Delaware No Drawing. Application June 22, 1944, Serial No. 541,646

1 Claim.

This invention is concerned with the problem of maintaining stress in steel in elevated temperature service. Those unfamiliar with the prior art knowledge of this problem may refer to pages 9'75 to 982 of The Making, Shaping and Treating of Steel, fifth edition, published by Carnegie-Illinois Steel Corporation.

Steel designed for elevated temperature service is made in various types, the present invention being concerned with the pearlitic, non-air-hardening plain carbon-molybdenum type containing about .50% molybdenum, such as is widely used in elevated temperature service Where the corrosion and oxidation conditions are sufiiciently mild to make its use most economical. Where the conditions involve more severe corrosion and oxidation it is common to use the more expensive type containing chromium so as to secure the corrosion and oxidation resistance afiorded by this alloy, the chromium increasing the cost of the steel and possibly detracting somewhat from its creep strength but this being offset providing the chromium prevents metal loss from the stressed steel through corrosion and oxidation to a degree sufficiently greater than if the chromium is absent. Therefore, in the latter type of steel, the chromium content is never less than 1% since lesser amounts do not afford sufiicient increase in corrosion and oxidation resistance to make its use economically practicable, the lowest chromium commercial grade containing about 1%% chromium. 'Thi chromium molybdenum type is air-hardening so its use is disadvantageous for this reason and is avoided whenever the corrosion and oxidation conditions make the use of the plain carbon-molybdenum type sound from a cost view-point.

Formerly, this plain carbon-molybdenum type was considered completely safe for use in stress at temperatures up to about 1100 F., since it was known to have an adequate creep strength within this temperature range. However, recently it has been found that with prolonged use at elevated temperatures the carbon in the steel graphitizes so as to produce planes of weakness with consequent unpredictable failure and this has introduced a serious hazard wherever such steel is used under stress in elevated temperature service. It is believed that this effect might prevail at temperatures as low as 850" F. with adequate time. and it is definitely present at temperatures above 900" F. It is apparent that there is a need for doing something about this problem while remembering that whatever is done must not increase the cost 01' the steel to that. of the more highly alloyed chromium-molybdenum type and should preserve the full creep strength of the plain carbon-molybdenum type and its non-airhardening advantage.

According to the present invention, from .15 to less than 1% chromium is added to the composition of the pearlitic, non-air-hardening type commercially containing from .08 to 20% carbon and from .45 to .65% molybdenum with the chr0- mium proportioned respecting the carbon content to fix substantially all the carbon in stable carbide form during elevated temperature service of the steel, the steel otherwise being of a composition suitable for its type. The addition of this small amount of chromium is impracticably uneconomical from the standpoint of fitting it to serve under conditions involving corrosion and oxidation normall demanding the air-hardening chromium-molybdenum type, since its service life under such conditions is not suificiently prolonged to cover the cost of the chromium addition, but it is practicably economical in the case of the type to which it is applied since its additional cost is is so greatly ofiset by its greater service life, it being remembered that this type is used under conditions where the corrosion and oxidation is not sufficiently severe to be a material criterion in selecting the type of steel to be used.

With this invention, stress may be safely maintained in steel within the temperature range of from about 850 to 1100 F. for a prolonged time period, where the conditions of corrosion and oxidation are not sufliciently severe to require or,

from a cost standpoint, permit the additional expense of the chromium-molybdenum type. As indicated, this is done by making a steel of the pearlitic, non-air-hardening type containing from .08 to 20% carbon and from .45 to molybdenum in conjunction with from .15 to less than 1% chromium which is proportioned respecting the carbon content to fix substantially all the carbon in the form of carbide that is stable within the defined temperature range, the steel otherwise being of a composition suitable for its type. Force is then applied to this steel to maintain stress' therein while, the steel is maintained within the defined temperature range. and no difiicult will be experienced from the problem of the carbon graphitizing and weakening the steel so that it fails under the stress.

Further advantages are that the amount of chromium used is so small that it; does not materially atlect the creep strength of the steel so that. the st el maintains. practically the same creep strength as if it were of the conventional plain carbon-molybdenum type, and it may provide an improved creep strength. Then too, the amount of chromium used is so small that it does not make the steel air-hardening, whereby it can be handled and processed in about the same manner as the type on which it represents an im.

provement The general composition of steel of the type here concerned is well known and the factors involved are explained by the previously mentioned text.

bodying the principle of the present invention may have a composition containing about .16% carbon, .45% manganese, .04% maximum phosphorus and sulphur, .25% silicon, .50% molybdenum and 50% chromium.

A representative'example of a steel em- We claim: A method of maintaining stress in steel within the temperature range of from about 850 to 1100 F.; including making a steel of the pearlitic, nonair-hardening type containing from .08. to 20% carbon and from .45 to .65% molybdenum in conjunction with from .l5to less than 1% chromium which is proportioned respecting the carbon content to fix substantially all the carbon" in the form of carbide that is stable within said temperature range, the steel otherwise being of a composition suitable for its type; and applying force to this steel to maintain stress therein while maintaining it within said temperature range. g 1

MARCUS A. GROSSMANN.

RICHARD F. MILLER. 

